XHTTP client: Define Request.GetBody() for packet-up so h2 can replay after GOAWAY (#6632)

https://github.com/XTLS/Xray-core/pull/6632#issuecomment-5430735467

---------

Co-authored-by: 风扇滑翔翼 <Fangliding.fshxy@outlook.com>

transport/internet/splithttp/config_test.go

@@ -1,9 +1,13 @@
 package splithttp_test
 
 import (
+	"io"
+	"net/http"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/xtls/xray-core/common"
+	"github.com/xtls/xray-core/common/buf"
 	. "github.com/xtls/xray-core/transport/internet/splithttp"
 )
 
@@ -77,3 +81,35 @@ func Test_GetNormalizedPath(t *testing.T) {
 		})
 	}
 }
+
+func Test_FillPacketRequest_GetBody(t *testing.T) {
+	data := []byte("hello xray")
+	payload := buf.MergeBytes(nil, data)
+
+	req, err := http.NewRequest("POST", "https://example.com/", nil)
+	common.Must(err)
+
+	config := &Config{}
+	config.FillPacketRequest(req, "sess", "0", payload)
+
+	if req.GetBody == nil {
+		t.Fatalf("Expected GetBody to be set")
+	}
+
+	first, err := io.ReadAll(req.Body)
+	common.Must(err)
+
+	if string(data) != string(first) {
+		t.Fatalf("Body mismatch. Format %q and %q are not equal", data, first)
+	}
+
+	body2, err := req.GetBody()
+	common.Must(err)
+
+	second, err := io.ReadAll(body2)
+	common.Must(err)
+
+	if string(data) != string(second) {
+		t.Fatalf("Replayed body mismatch. Format %q and %q are not equal", data, second)
+	}
+}